(1) Field of the Invention
The present invention relates to an image forming apparatus, such as a digital color copying machine and a color facsimile machine, which reproduces a full-color image according to image data obtained by reading an original image.
(2) Related Art
In a copying machine which reproduces an image according to image data obtained by reading an original with a scanner, the digital image data of red (R), green (G), and blue (B), obtained from an original, are transformed into color reproduction data of cyan (C), magenta (M), and yellow (Y) by an image signal processing unit. According to the color reproduction data, an image is formed on a printing paper by a printer.
During the data processing, however, image noise is caused by low scanning accuracy, poor color reproducibility of the printer, and other factors, and that is why an image forming machine needs a correction unit.
In a full-color reproduction image, unevenness is very noticeable in a uniform density region, where smoothness is an important factor as well as resolution. If the resolution is emphasized, however, the unevenness stands out even more in a uniform density region and makes the whole image unnatural. If smoothness is emphasized, the edge portion is blurred and the result is very poor resolution.
Japanese Laid-Open Patent Application No. 4-342370 discloses an image processing method in which the edge portions and the uniform density regions are discriminated depending on changes in lightness. Here, the image data of the edge portions are subjected to edge emphasizing by an MTF (modulation transfer function) correction unit to reproduce a clearer image, while the image data of the uniform density regions are subjected to smoothing to obtain a smoother image. By doing so, both resolution and smoothness are improved, and image noise is reduced.
The conditions for MTF correction are optimized at the time of assembling or shipping of the copying machine.
The actual image quality, however, is not always as good as expected, despite the fact that the MTF corrective conditions have already been optimized at the time of assembling or shipping.
This is mainly because each component deteriorates with time. More specifically, the deterioration is due to declines in the sensitivity of the CCD image sensor, the irradiation by the exposure lamp, the sensitivity of the photosensitive drum, and the sensitizing ability of the sensitizing charger. The influence from a change in the surroundings and the characteristics of each copying machine may also be the cause of the deterioration.
In such case, optimizing the MTF corrective conditions is difficult, because it requires skill and complicated modulation processes.